1. Field of the Invention
The present invention relates to a method of manufacturing an optical element using a mold and to an optical element manufactured thereby. The present invention is particularly suitable for manufacturing, for example, a diffraction optical element of a stack structure with a grating structure for allowing the light beams in the region of the wavelength used to concentrate on a specific order.
2. Related Background Art
Hitherto, the chromatic aberration of an optical system has been corrected by combining optical elements made of glass materials different in dispersion. Further, the articles published in SPIE Vol. 1354 on pages 24-37 disclose a method of correcting chromatic aberration using a diffraction optical system.
When adding a diffraction plane having a diffraction effect to an optical system having spectral characteristics, it is important to keep high the diffraction efficiency in the region of the wavelength used. However, with the diffraction optical system, as to a light of an order other than the design order, the more the order becomes different from the design order, the larger the diffraction angle will be and the larger the difference in focal length will be. This will result in defocus, and especially when there exists a light source with a high luminance, side lobe may sometimes occur.
On the other hand, by constructing a diffraction optical element of a stack structure having two or more layers, considerable improvement in quality of images and information may be expected and improvement in optical performance is made possible. The stacked diffraction optical element having two or more layers can be formed by superposing and joining a plurality of optical members each having a diffraction grating. Further, the method of manufacturing such optical members includes, for example, a method of filling a molding material into a mold having a pattern corresponding to a diffraction grating formed therein, transferring the pattern to the molding material, and then separating the molding material from the mold.
On the other hand, as the method of forming a pattern on a surface of a mold, there has been known a method using a typical fine-processing technique, such as photolithography of a semiconductor manufacturing process or a precision cutting technique with a diamond tool. By molding plastic or glass with a mold having a pattern formed by such processing techniques, the optical members described above can be manufactured.
However, the stacked diffraction optical element allows improvement in optical performance as described above, but still has problems such that its manufacturing method is very complicated, its costs are high, and it is difficult to put into practical use, as long as the conventional methods are used as such, though the details will be described below.
In the stacked diffraction optical element described above, the optical members for stacking themselves must be produced with a high accuracy. As a technique for producing replicas of optical members, a replica molding method using a photo-setting resin has hitherto been preferably used on account of its high transferring property, profile irregularity and convenience. This method makes it possible to transfer a highly fine pattern from a mold having the highly fine pattern formed therein by photolithography technique, to a molding material, and therefore, is an important technique still. As for the transferring property, though they are affected by the amount of cure shrinkage of the material itself, there have been proposed various techniques.
For example, in Japanese Patent Application Laid-Open No. 3-79314, there is proposed a mold release technique for use in the molding method. This technique, however, gives rise to a problem, as described below, when the depth of the unevenness of a fine pattern is large. A stacked diffraction optical element is literally formed by joining two or more optical members each having a diffraction grating. Therefore, considering the weight reduction and miniaturization of the products in which the stacked diffraction optical element is used, each optical member is required to be thin. However, in the conventional replica molding method described above, mold release is performed in such a manner as to pull up a glass substrate of a diameter larger than that of the mold; therefore, during the progress of mold release, a very large amount of warp (deformation) in the entire element will be generated.
In the molding of a graded index lens having a spherical or a spherical surface, for example, the shape of the molded form is not affected very much by mold release; however, in the molding of a diffraction optical element with unevenness of a fine pitch on its surface, the release angle due to warps and deformations produces a force which may knock the fine molded form down, whereby the molded form is deformed or damaged. Since the magnitude of the deformation or the damage mainly depends on the adhesion between the molding material and the mold material, in order to lower the adhesion, a method can be adopted in which a release agent is applied onto the mold; however, considering the occurrence of the disorder of the fine molded form and difficulties of the maintenance during mass production, the method still has problems.
In the following, the above problems will be described in detail. Referring to FIG. 1, there is shown a schematic sectional view illustrating the prior art method of manufacturing an optical element. In the same figure, reference numeral 1 denotes a mold. In the surface of the mold 1, there is formed a fine pattern corresponding to a diffraction grating which is to be formed finally.
When manufacturing an optical element, first photo-setting resin is dropped onto the mold 1 and then a glass substrate 2 is put thereon so as to spread the resin entirely on the mold to give a desired thickness. Then, the photo-setting resin is irradiated with ultraviolet light via the glass substrate 2 to be set so as to form a resin layer 3. On the bottom surface of the resin layer 3, the pattern of the diffraction grating formed in the mold is transferred. Lastly, ejector pins 4 provided just around the mold 1 are moved in the direction shown by arrows D so as to push up the peripheral portion of the glass substrate 2, whereby the resin layer 3 together with the glass substrate 2 is separated from the mold 1.
As described above, when separating an optical member from a mold by applying a force thereto, the pattern as transferred to the optical member may be deformed or damaged. This tendency is remarkable when forming an optical member with a diffraction grating having a concave lens effect. Referring to FIG. 2, there is shown a schematic sectional view illustrating a state in which an optical member with such a diffraction grating having a concave lens effect (i.e., concave diffraction grating) is separated from a mold 1. For an optical member with a diffraction grating having a convex lens effect (i.e., convex diffraction grating), in other words, with a concave-type mold necessary for molding thereof, when allowing the release of the optical member to proceed from the outer side of the mold to the center of the mold, the separation of the optical member from the mold progresses while keeping the molded resin grating from colliding against the grating shape of the mold. However, for an optical member with a concave diffraction grating, in other words, with a convex-type mold, when allowing the release of the optical member to proceed from the outer side of the mold to the center of the mold, just like the above case, the grating of the resin layer 3 molded as shown in FIG. 2 will collide against the grating shape of the mold, whereby the grating shape of the molded piece is significantly deformed or damaged. In FIG. 2, an arrow A denotes the direction in which the separation proceeds. It is known that such a diffraction grating causes a lowering in diffraction efficiency and a flare to occur, thus significantly affecting the image quality.
It is, therefore, an object of the present invention to solve the problems of the prior art described above and provide a method of manufacturing an optical element which allows a stable mass production without causing deformations and damages during the mold release.
According to a first aspect of the present invention, there is provided a method of manufacturing an optical element, comprising the steps of:
providing a molding material onto a mold;
giving a local temperature difference to an interface between the mold and the molding material to separate the mold and the molding material from each other in an area; and
successively enlarging the area of separation made by the temperature difference to entirely separate the mold and the molding material from each other.
According to a second aspect of the present invention, there is provided a method of manufacturing an optical element having in a surface thereof a fine pattern with a concave lens effect, comprising the steps of:
providing a molding material onto a mold having in a surface thereof a configuration corresponding to the fine pattern;
putting a substrate on the molding material;
giving a local temperature difference to an interface between the mold and the molding material at a peripheral portion of an optical element to be manufactured, to separate the mold and the molding material from each other in an area;
enlarging the area of separation made by the temperature difference successively from the peripheral portion to a center portion of the optical element such that the substrate warps convexly relative to the mold and only the center portion remains unseparated with the peripheral portion being separated;
bringing an ejector pin projecting from the mold side into contact with a peripheral portion of the substrate; and
heating an interface between the mold and the molding material at the center portion of the optical element to entirely separate the mold and the molding material from each other.
According to a third aspect of the present invention, there is provided an optical element manufactured by the method as set forth above.
According to a fourth aspect of the present invention, there is provided an optical system comprising a plurality of optical elements including the optical element as set forth above.
According to a fifth aspect of the present invention, there is provided an optical device for forming an image using the optical system as set forth above.